1. Field of the Invention
The present invention relates generally to cold-pilgering of thin-walled metallic tubing and, more particularly, is concerned with a ferrous metal detector for detecting a broken mandrel in a cold-pilgering operation.
2. Description of the Prior Art Cold-pilgering is a conventional tube forming operation by which a tube is simultaneously advanced over a stationary mandrel and compressed using two opposing roller dies resulting in the reduction of the cross-sectional area and in elongation of the tube. Representative of prior art pilgering machines are the ones disclosed and illustrated in U.S. Pat. No. (3,416,346) to Arrington, U.S. Pat. Nos. (3,487,675 and 3,690,850), to Edstrom et al, U.S. Pat. No. (4,090,386) to Naylor et al, and U.S. Pat. No. (4,233,834) to Matinlassi.
Typically, the input tube is reduced and elongated to the final tube by passing through a succession of stations of the cold-pilgering machine with each station being composed of a stationary mandrel and roller die set. Reduction is effected in both the diameter and wall thickness of the tube by means of the tapered shape of the mandrel and the circumferential tapered shape of grooves in the dies which embrace the tube from above and below the mandrel and roll in a constant cycle back and forth along the tube. Between each cycle of die movement, the tube is advanced and rotated incrementally along the mandrel. The mandrel prevents the tube from collapsing under the force of the roller dies while at the same time dictates the inner diameter of the tube.
Although the mandrels and roller dies are fabricated from high strength steel, a limiting factor in the cold-pilgering operation is the need for frequent replacement of mandrels and roller dies. Mandrel replacement is required when the steel mandrels become overstressed and break from severe operating conditions and occasional bending moments imposed thereon by tube eccentricity or slight misalignment. Roller die replacement is frequently required due to occurrence of surface cracks, fretting and spalling in the die grooves of the steel dies as a result of severe operating conditions of the pilgering machine. The invention of the patent application cross-referenced above is directed to increasing the longevity of pilger tooling, i.e., mandrels and roller dies, to improve the productivity and efficiency of the pilgering machine by providing high strength cemented carbide pilger tooling.
The occasional breaking of a mandrel can cause defects in the tube being formed or, more importantly, cause damage to the roller dies as they compress against the sharp edge of a broken mandrel. Once damaged, the dies must be replaced and repaired, resulting in costly downtime. Thus, it is advantageous to be able to detect a broken mandrel immediately after the break occurs so as to avoid the resultant damage to the dies.
Heretofore, a tuned oscillator circuit has been used to detect a broken mandrel. The circuit is tuned by the operator to calibrate or zero out the effect of the presence of the tube being formed so that the presence of a broken mandrel can be detected. However, interference from the tubing material still causes inaccuracy in the readings. These inaccuracies increase with the wall thickness of the tubes.
Consequently, a need exists for a way to consistently and quickly detect, regardless of tube wall thickness, broken mandrels during the cold-pilgering operation.